Iron allotropes

Soft white, ductile metal high-purity metal is very ductile at ordinary temperatures occurs in three allotropic forms (i) body-centered cubic form, alpha iron stable up to 910°C, (ii) face-centered cubic form, gamma iron occurring between 910 to 1,390°C, and (iii) body-centered delta iron allotrope forming above 1,390°C. Density 7.873 g/cm at 20°C melting point 1,538°C vaporizes at 2,861°C hardness (Brinell) 60 electrical resistivity 4.71 microhm-cm at 0°C tensile strength 30,000 psi Poisson s ratio 0.29 modulus of elasticity 28.5 X 10 psi thermal neutron absorption cross-section 2.62 bams velocity of sound 5,130 m/s at 20°C. 

Table 3.2. Uiut Cell Dimensions of Iron Allotropes and Fe—C Alloys ...

For iron allotropes, why is the solubihty of carbon greater in the austenite phase, relative to the ferrite phase ... 

Between the pure iron allotropes and the intermediate phase cementite, a number of solid solution regions occur. The extent of these depends on the... 

Table 2.3. Physical properties of four iron allotropes and high-temperature forms...

The metal looks like iron it exists in four allotropic modifications, stable over various temperature ranges. Although not easily attacked by air. it is slowly attacked by water and dissolves readily in dilute acids to give manganese(II) salts. The stable form of the metal at ordinary temperatures is hard and brittle—hence man ganese is only of value in alloys, for example in steels (ferroalloys) and with aluminium, copper and nickel. 

Gobalt is a brittle, hard metal, resembling iron and nickel in appearance. It has a metallic permeability of about two thirds that of iron. Gobalt tends to exist as a mixture of two allotropes over a wide temperature range. The transformation is sluggish and accounts in part for the wide variation in reported data on physical properties of cobalt. 

Pure iron is a fairly soft silver/white ductile and malleable moderately dense (7.87 gcm ) metal melting at 1,535 °C. It exists in three allotropic forms body-centered cubic (alpha), face-centered cubic (gamma), and a high temperature body-centered cubic (delta). The average value for the lattice constant at 20 °C is 2.86638(19)A. The physical properties of iron markedly depend on the presence of low levels of carbon or silicon. The magnetic properties are sensitive to the presence of low levels of these elements, and at room temperature pure iron is ferromagnetic, but above the Curie point (768 °C), it is paramagnetic. 

Muller, J. Joubert, J.C. (1974) Synthese en milieu hydrothermal et caracterisation de Voxyhydroxyde de vanadium V OOH et d une nouvelle variete allotropic du dioxide VO2. J. Solid State Chem. 11 79—87 Muller, J.P. Bocquier, G. (1986) Dissolution of kaolinites and accumulation of iron oxides in lateritic-ferruginous nodules Mineralogical and microstructural transformations. Geoderma 37 113-136... 

A table of crystal structures for the elements can be found in Table 1.11 (excluding the Lanthanide and Actinide series). Some elements can have multiple crystal structures, depending on temperature and pressure. This phenomenon is called allotropy and is very common in elemental metals (see Table 1.12). It is not unusual for close-packed crystals to transform from one stacking sequence to the other, simply through a shift in one of the layers of atoms. Other common allotropes include carbon (graphite at ambient conditions, diamond at high pressures and temperature), pure iron (BCC at room temperature, FCC at 912°C and back to BCC at 1394°C), and titanium (HCP to BCC at 882°C). 

The allotropy of elemental iron plays an important role in the formation of iron alloys. Upon solidification from the melt, iron undergoes two allotropic transformations (see Figure 2.9). At 1539°C, iron assumes a BCC structure, called delta-iron (5-Fe). Upon further cooling, this structure transforms to the FCC structure at 1400°C, resulting in gamma-iron (y-Fe). The FCC structure is stable down to 910°C, where it transforms back into a low-temperature BCC structure, alpha-iron (a-Fe). Thus, 5-Fe and a-Fe are actually the same form of iron, but are treated as distinct forms due to their two different temperature ranges of stability. 

Carbon is soluble to varying degrees in each of these allotropic forms of iron. The solid solutions of carbon in a-Fe, y-Fe, and <5-Fe are called, respectively, ferrite, austenite, and 8-ferrite. So, for example, the single-phase region labeled as y in... 

Three allotropic forms of iron are known (I) alpha irott. which is present below 769 C t2) grimnitr iron, which exists between 906 and 1.4(14 0, uid (3) delta imn, which occurs between 1,404 and l,536"C, On slow cooling, die reverse changes occur, but may be slowed or partly or entirely prevented in the presence ol alloying elemenls. 

When an element can exist in more than one physical form in the same state it is said to exhibit allotropy (or polymorphism). Each of the different physical forms is called an allotrope. Allotropy is actually quite a common feature of the elements the Periodic Table (p. 136). Some examples of elements which show allotropy are sulfur, tin, iron and carbon. 

Steel, as is well known, differs from iron by the presence of a certain amount of carbon, which induces the iron, when cold, to persist in its allotropic state. This appears to be due to a carbide of iron mixed with the excess of iron in the steel. The compound has been found as a meteoric mass it has been named cohenite, and has the formula FegC. On treating steel with dilute acetic acid, the same substance remains as a black powder. Its formula is similar to that of manganese carbide, MngC. 

Kaufman was with the Lincoln Laboratory at M. I. T. from 1955 to 1958, ManLabs from 1958 to 1988, and Alcan Aluminum Corporation from 1988 to 1996. Kaufman derived expressions for thermochemical lattice stabilities for allotropic and nonallotropic elements. He provided an early model for the thermodynamic description of iron, including magnetic contributions, and he was the founding editor of the CALPHAD journal. Kaufman is now a consultant and lecturer in the materials science and engineering department at M. I. T. 

The foregoing results are usually interpreted as indicating that iron is capable of existing in four allotropic modifications, designated respectively as a, / , y, and 8 ferrite, the points A2, A3, and A4 representing their transition temperatures (that is, the temperatures at which the... 

Benedicks, J. Iron Steel Inst, 1912, IL, 242 1914, L, 407 Carpenter, ibid, 1913, I., 315. Le Chatelier (Rev. Metallographie, 1904, I., 213) appears to have been the first to suggest that the A2 point is not connected with allotropic change. 

Figure 3.9. Simplified scfiematic of tfie transformation from BCC to FCC, exfiibited between the three allotropes of iron. Comer atoms have been omitted for clarity.

In general, the density of interstitial solid solutions is given by Eq. 6. Since the change in volume is usually more significant than the increase in number of unit cell atoms, interstitial solids usually exhibit a decrease in density, relative to the pure allotrope. For instance, the density of pure iron (7,874 kg m ) shows a significant decrease upon interstitial placement of carbon in cast irons (ca. 7,400 kg m ). 

The complex binary phase diagram for the Fe-C system is shown in Figure 3.10, and illustrates a number of important transitions. In particular, as the temperature is increased from ambient to its melting point, pure iron exhibits a variety of allotropic changes. At room temperature, the ferrite form is most stable conversion to austenite... 

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